Thank you for your question.
Virginity cannot be determined by presence or absence of bleeding or the state of the hymen. In girls the hymen can be broken by simple things like sports, cycling, vigorous exercise or masturbation. The flexibility of the tissue determines if the trauma of intercourse causes bleeding. If she says she is a virgin you might wish to ask yourself why you don't believe her. Has anything happened other than during sex that caused you to have a doubt.